We all know that Sempervivum are tough plants given the right conditions, which leads to the question, what are the right conditions?

To answer this question there have been many experiments done with these tough plants to find out how they grow. My goal here is to explain some of the findings so we may understand our Sempervivum better. Heat, light, temperature all factor into how semps breathe, take in nutrients and grow. So my first post will be some definitions and some basic simplified plant physiology explanations, and I will edit and add to this post as time allows

Before I start I just want to say I am not a plant biologist, just a gardener who likes to know how things work so If someone more knowledgeable knows something thats not correct, feel free to point it out. The goal here is to help everyone understand their plants better. Feel free to tree mail me with opinions, changes or corrections!

Some definitions to start:
Stomata are the tiny pores in plant leaves that allow for gas exchange. They are usually found on the underside of plant leaves.

Photosynthesis is the process by which plants and other things make food. It is a chemical process that uses sunlight to turn carbon dioxide and water into sugars the cell can use as energy. A byproduct of photosynthesis is oxygen.

There are 3 ways that plants can convert light and CO2 into the sugars and carbohydrates needed to grow, but for our Sempervivum, only 2 of these ways are of interest to us. Here is a basic very simplified explanation of each.

C3 photosynthesis is the pathway that most plants use. C3 plant take open their stomata during the day, take in CO2. Using light energy they split off the hydrogen atoms from a H20 or water atom and combine the hydrogen and carbon dioxide into the sugars and carbs needed for growth. They give off oxygen as a byproduct. If there is a shortage of CO2 or water growth stops. This type of photosynthesis uses alot of water, because having the stomata open all day during the hot part of the day, causes the plant to lose alot of water due to evaporation. Most lawn grass and trees use this type of photosynthesis.

Crassulacean Acid Metabolism or CAM photosynthesis is what most succulents including Sempervivum use(most of the time). It differs from C3 in that CAM plants open their stomata take in CO2 at night and the cool parts of the day and store it in special cells. This allows them to close their stomata during the heat of the day to conserve water. The CO2 is stored in special cells as an acid, which is broken back down and the CO2 used during the day for C3 photosynthesis. It prevents water loss during the day, which allows CAM plants to be much more water efficient, but because they only take in CO2 at night and the very first part of the day, once it is used up growth stops, or they may open their stomata back up later in the afternoon to take in more CO2, so in general CAM plants are much slower growing. How much CO2 is used and when stomata are opened and closed for CAM plants to take in more CO2 are dependent on temperature, daylength and light levels. CAM photosythesis allows succulent plants to function is desert and alpine conditions with hot dry days and cool nights where other plants would wither and die.

As a final thought plants do have respiration. Plant cells still need oxygen to breathe just like you and I and when they breathe they give off CO2, but these amounts are much smaller than the photosynthetic process of converting CO2, water and light into sugar and giving off O2 in the process.

After reading the previous post, how do Sempervivum conduct photosynthesis? For much of the time Sempervivum are CAM plants as you may have guessed by now. The CAM mechanism lends itself to wide swings of temperature, dry conditions and copious amounts of sunlight, which is part of why Sempervivum, "live forever." The CAM process has 4 stages. Here is a general description of what goes on in each stage:
STAGE 1, Dark period---stomata open and take in CO2 which is fixed in the plant as malic acid.
STAGE 2, First 2 hours of light---stomata still open and taking in CO2, malic acid still being fixed in plant cells, then the stomata slowly close.
STAGE 3, Morning into afternoon---stomata are closed and plant breaks down malic acid to use the stored CO2 for photosynthesis. This is when CAM plants grow. Once all the malic acid is broken down and the stored CO2 is used up, we enter the next stage.
STAGE 4---afternoon until dark, after the plant uses the stored CO2, the stomata may open back up and the plant takes in CO2 and uses C3 photosynthesis for the remainder of useful light.

There are certain environmental conditions that experiments show Sempervivum do not use CAM, instead they act like C3 plants, complete with the greater water loss from having stomata open during the day.

So for the most part think of your Sempervivum like a storage battery, storing CO2 at night and using it during the day, but if not enough can be stored, or light levels high enough that it is used up quicker and the plant opens up its stomata in the afternoon, it opens itself up to much greater water loss, which is why it seems when growing Semps that they can take either heat, or alot of sunlight but not both. This is because it causes the semp to use up its CO2 faster and they open the stomata before the day has cooled and now lose alot of water to evaporation.

Sempervivum can survive in a broad range of temperatures, they are cold hardy to USDA Zone 4, which is an average minimum temparature of -30F, and some have successfully kept Sempervivum in Zone 3 which is 10 degrees colder! Experiments show they can tolerate short term temperature bursts all the way to 143F and still survive. In one experiment Semps were kept at 122F and it took 6 days before the plant was damaged.

Survival is one thing, but as gardeners, we want to know what makes them thrive! As alpine plants they have evolved to deal with little water and wide temperature swings from day to night, in fact the best conditions are warm days and cool nights. The metabolism of Semps affects how they can handle temperature. Semps in the summer growing period were damaged by a drop to 23F, but if the Semps were kept at 32F for 3 days, they werent damaged until the temp dropped to 14F. If the period they were kept at 32F was extended longer, they could withstand temperatures colder than -13F with no harm. This means that if the Semps metabolism is slowed down by cold, they are able to withstand extreme cold with no harm.

An extensive study was done to see how temperature affected photosynthesis in Sempervivum. Remember that if they arent processing CO2, then they are not growing, or even dormant. Semps show CO2 uptake in the light, in temperatures from 28F to 113F. They show CAM fixation of CO2 into malic acid in temperatures from 28F to 95F, although at night temperatures above 68F, they quit taking in CO2 and instead only convert CO2 from cell respiration into malic acid. High night temperatures seem to slow growth considerably. Even when well watered, Sempervivum still use CAM photosynthesis in most conditions, to availability of water does not change the type of photosynthesis they use. Temperature can make them change the mode of photosynthesis they use. At daytime temperatures colder than 50F, or nighttime temperatures warmer than 95F, Semps to act like C3 plants. At 50F or below this is not usually a problem, but in a heatwave, if they dont get cool nights, and go into C3 mode full time, they will probably experience great waterloss.

So what does all this data mean? One thing to keep in mind is that Semps in direct sun can be 15F warmer than outside air temperatures, so keep that in mind for the daytime figures.

Summarizing all that scientific mumbo jumbo:

-30F to 27F Sempervivum not processing CO2, no growth, if temperature stays below this, dormancy. Not much need for water, but snowcover will help stop water loss from cold winds.

27F to 50F(daytime) Sempervivum using C3 photosynthesis but growth is slow, meaning they use some water and take in CO2 during the light instead of at night.

50F to 113F(daytime) Sempervivum in ideal growth mode using CAM photosynthesis as long as nights are cooler than daytimes and are 28F to 68F. If nights are warmer than 68, growth slows down because not as much CO2 is fixed at night, and it may also allow an early start to phase 4 CAM causing greater water loss in the late afternoon. Too much sun in temperatures warmer than 85F may cause the plant to use up its stored CO2 too quickly and start phase 4 CAM earlier in the afternoon as well. This is why Semps seem to melt in high heat and high light. More watering is needed to compensate for this water loss. If nighttime temperatures stay above 95F, the sempervivum will go into C3 photosynthesis and at such high temperatures, lose even more water and probably die off.

113F to 122F Sempervivum cant process CO2 at these temperatures and will go dormant.

122+ More than 5 days at these temperatures causes permanent damage/death.

It seems that Sempervivum definitely do best spring/early summer, and fall, even winter plants for those of us who dont get snow up to their eyeballs, and that the most trying time are hot summers, especially if the nights dont cool.

We have seen how temperature affects how Sempervivum grow but until now have ignored the most important ingredient for plant growth, LIGHT!!! Most sites will tell you that they like alot of light, but what does that really mean? How much light? Is too much light possible?

Sempervivum need light to conduct photosynthesis whether they are in C3 mode or CAM mode, because light drives the whole photosynthetic action. Before I discuss findings on light requirements for Semps, I want to explain light and light levels.

Sunlight is composed of many different wavengths of light, that in total make up what we see as "white" light. For plants however, not all of the light is useful for photosynthesis. The two wavelengths that plants use for photosynthesis are the red and blue wavelengths of light. Some other colors are used for other reactions inside plant cells, and the light that is notbas useful to a plant is green! Thats why most plants are green, they are reflecting the green light that they do not need for anything. By chance, human eyes are the most sensitive to green light. But growing outdoors, we dont have to concern ourselves with light colors, as the sun has all the light needed to grow Sempervivum, but what does matter is how much light they get.

The term lux is a measure of how bright light is. It is equal to one lumen per square meter. To get an idea of outdoor light levels here is a guide:

Humidity, dust, clouds etc, will cause these numbers to go down a bit, but suffice it to say the sun is pretty bright. A well lit office space is typically 300-500 lux by comparison.

The more light available, the faster photosynthesis can occur up to a certain point. A plant is said to be light saturated when more light doesnt make photosynthesis go any faster. In other words, when a plant is light saturated, light is no longer the limiting point to growth.

The light saturation point for Sempervivum is 41,000 to 55,000 lux when they are in active CO2 uptake in the light (C3 mode generally) and its around 27,000 lux when a Sempervivum is using CO2 from breaking down malic acid (CAM) mode. Light any brighter than this no longer aids growth. So bright sun on a clear day at noon is about double the light they require in C3 mode and when acting normally, they need a about a third more light than bright shade.

Does additional light have any detrimental effects? On a hot day it can, if the Semp uses up its stored CO2 and enters phase 4 of CAM and opens the stomata when its still hot, it can lose alot of water to evaporation. What can we do to prevent this from happening? Cut the light levels, or the lighting period! Less light slows photosynthesis. Unless the Sempervivum are stretching for light and looking etiolated, shading them is the answer. High Heat+High Light=Semps opening stomata and losing water.

Does additional light have any desired effects? This is a tricky question. More light than the light saturation point can not make the Sempervivum grow any faster, but it does have other effects. The plants themselves try to self regulate by opening and curling up the leaves in response to light levels. High elevation desert cactus that are not shaded evolved a way to shade themselves, which is white fur. The webbing on S. arachnoideum does much the same thing. Another effect that high light levels have is on coloration. Remember that plants are generally green, because they reflect the green light that they do not use or need. Genes determine the bulk of leaf color, but some plants can change leaf color in response to excess light. Although I can not find any research on Sempervivum to back this up, I think Sempervivum may redden up in response to excess light in the colder months when their metabolism slows. Many plants have a similar response to excess light, when they turn red they reflect the red light they do not need for photosynthesis.

As with anything, there is a balance to maintain, too little light and growth is slowed, and too much light in the heat, Semps melt because they lose water from evaporation, our job as gardeners is to find the happy medium.

Bigtrout said:... but if not enough can be stored, or light levels high enough that it is used up quicker and the plant opens up its stomata in the afternoon, it opens itself up to much greater water loss, which is why it seems when growing Semps that they can take either heat, or alot of sunlight but not both. This is because it causes the semp to use up its CO2 faster and they open the stomata before the day has cooled and now lose alot of water to evaporation.

So this is why heat and high sun exposure is so dangerous to our semps...which is why it's so important to move them into shade on a hot sunny day! And sometimes I bring the shade to them by draping with shade cloth if they can't be moved.

Exactly, from my reading, they seem to be geared exactly for their alpine environment. Living in pockets on rock outcroppings they probably dont get sun all day, but are geared to take full advantage of intense direct sun for a few hours. Keep in mind that the sun at elevations where they evolved is even more intense than sun around sea level, but being tucked among the rocks means they probably dont get it all day.
This also may mean that during intense heat+sun they need more water than usual.

Found this info on a grower bulletin which would tie in nicely with Brians info:

Soil: Any well-draining, normal greenhouse soilless media will suffice. The breeder uses SunGro852, which is the same as Fafard 52 soilless media.

• Tagging: The breeder highly recommends NOT putting tags in at the time of fall planting. A tag creates a “water arch” when watering and can also lead to small dots of mildew prior to shipping next spring.

• Watering: Keep freshly potted chicks on the moist side during their active fall growth cycle. Chicks put on a tremendous amount of growth during the cool months of September, October and November. The breeder hand waters all Chick Charms for best results. However, several varieties require less water and should be kept on the drier side. These drier growing varieties include Key Lime Kiss, Cotton Candy and Fringed Frosting.

• Fertilizing: Fertilize heavily during the fall growth spurt cycle with 200 to 300 ppm general all-purpose fertilizer or 200 ppm of a good Calcium-Magnesium (CaNO3/MgNO3) based fertilizer.

• Temperatures: Keep fall greenhouse temps at 65F (18C) heat/75F (23C) vent/80F (26C) fans until December 1. Then provide a gradual temperature reduction over a two-week period. Finally, hold them at 34 to 40F (1 to 4C) for the rest of the winter.

• Vernalization: Chick Charms do require an eight-week cold/vernalization period to obtain their best coloration the following spring. Keep at 34 to 40F (1 to 4C) for an eight-week period beginning December 1 and running through late January.

Spring growing

• Fertilizing: Use light to no fertilizer in the spring on Chick Charms. You can use minimal 50 ppm fertilizer in the spring with every third watering. However, too much nitrogen can prevent bright coloration and cause green, lush growth instead.

• Watering: Keep plants on the moderately moist side. This helps encourage chicks to “plump” up and fully fill the pots.

• Temperatures: Increase temps to 60F (15C) during the night and maximum 75F (23C) during the daytime.

• Insects: Watch for root aphids, which are a white, cottony mass on the root system. If detected, treat with a Safari drench at 9 oz./100 gal.

Miscellaneous production notes

• Always water ONLY in the cool morning hours prior to 10:00 a.m. Later watering in the heat of the day can cause instant rot.

Prudence, indeed, will dictate that Governments long established should not be changed for light and transient causes; and accordingly all experience hath shewn, that mankind are more disposed to suffer, while evils are sufferable, than to right themselves by abolishing the forms to which they are accustomed.

Great info Brian. Thank You.
Photo of Semp growing in its native habitat in mountains of Italy. Notice the grass growing with the semp. I have seen this in many (but not all) photos of semps. in habitat. Some are right out there on the rocks but I think that even some dried up grass provides some shade and protection against the strong sun. We try to keep the "weeds" out of beds and pots
Crawgarden, that is an interesting article on greenhouse growing. This is how Chris Hansen is doing it in a very controlled environment. However, that soil mix is death to semps grown outside where they get rained on. It holds way way too much water. It works in a controlled environment where water application is measured very carefully and can be held back during cold winter weather. Most of us are growing outside in beds so we need a very free draining soil, especially in the PNW.

Prudence, indeed, will dictate that Governments long established should not be changed for light and transient causes; and accordingly all experience hath shewn, that mankind are more disposed to suffer, while evils are sufferable, than to right themselves by abolishing the forms to which they are accustomed.

If you read about the CAM rhythm again, seems that by the natural timing, Semps would love Morning sun and partial shade in the afternoon.

@ricos One study thats part of the info im using to write this talks about Semps being generally found in very shallow soil as in 1.25 to 2.5 inchs so that would be extremely well draining but yes the wild ones are usually near something for some shade part of the day, a rock, deadfall or near other taller plants etc.

This was a great read. Thank you so much Brian for putting it together. I want to read it again this evening and take some notes.
Rj I also found the part about fall growth interesting. I have always felt that spring and fall were great times to start new plantings.